Moulds for precision casting



amazes MOULDS FOR PRECISION CASTING Adam Dunlap, Gosforth,Neweast-le-upon-Tyne, England, assignor to Rolls-Royce limited, Derby,England, a British company No Drawing. Application August 24, 1953,,Serial No.. 376,243

Claims. priority, application Great Britain September 11, 1952 8 Claims.(l. 22-496) In what has now become recognised as the conventional methodof making moulds for investment casting using an expendable pattern suchas wax or plastic, the mould is produced by a two stage process. Thefirst involves a relatively thin coating of the pattern assembly byspraying, painting or dipping and the second the final investment of thecoated pattern in a suitable container by a slurry or refractorysuspended in a liquidbinding medium, the final consolidation of themould being obtained by vibration.

When the liquid binding medium has hardened by chemical action, theexpendable pattern is melted out by heating at 100150 C. for severalhours. Following this, the mould is gradually raised to a temperature inthe range 850-1100 C. to remove completely the last trace of wax andvolatile constituent of the binder medium. This requires several hoursheating.

The process of consolidating the slurry by vibrating requires from 1 to4 hours, thus where a high mould output is required, many vibratingtables are necessary. 8 to 10 hours are generally required for both theremoval of the main bulk of the expendable pattern and for the heatingto the high temperature. Again where a large output is being handled,large ovens and mould firing furnaces are needed.

I am aware that there has been previously described a process forprecision casting by the lost wax. process in which a wax pattern of theobject to be cast is provided with an initial refractory coatingconveniently applied by spraying, and then with a multiplicity ofsuperimposed dip coatings such as to result in the pattern beinginvested with a substantial shell of refractory material namely of theorder of from A inch to /2 inch thick (according to the size and shapeof the wax pattern): after formation of the shell investment it iscontemplated to remove the wax pattern, preferably by the use of asuitable hot solvent, and then to complete the forming of the mould bysurrounding the shell with a supporting packing of dry sand.

According to this invention a multi-layer shell mould is formed aroundan expendable pattern by a process which includes dipping alternately inan acid and an alkaline solution of a sodium silicate, and including afinely ground refractory material suspended in the solution, thenormality of the alkaline solution being greater than that of the acidsolution, each successive, single dipping being followed by theapplication of a stucco material to the Wet mould.

By this means a sufiiciently rigid shell can be. formed around thepattern to form a mould which can be used with an investment which ismuch simpler, cheaper and more quickly provided than is the conventionalmethod and will have other advantages as below stated.

The normality of the alkali solution must be considerably greater thanthe normality of the acid solution to ensure rapid gelling when the twoare brought together on the shell. I prefer to keep the ratio about2-311.

ited States Patent Preferably before the application of the layers of,the shell proper the pattern is given an initial refractory coatingbydipping in or spraying with a; solution of the type used in theconventionalmethod.

The shell mould may be built: up as follows: An initial coat is formedon the pattern by spraying with or dip.- ping in a refractory solutionofknown composition. Suitable solutions to form the initial coat are asfollows;

(a) Applied by spraying.

(1) Soap solution (1%) ml' 3-20 Sodium silicate ml I60 Hydrochloricacid, 10% ml 36 Flint powder "gm" 1000 (Z) Ethyl silicate containingpiperidineuml u 160 Methylated spirits ml-' 320 Flint-powder gn1' 1000(3) Ethyl silicate ml 70 Methylatedspirits ml Q5 Water m- 5 Flint powdergm... I000 (b) Applied by dipping (1)- Water ml 2160 Sodium. silicate..mL.. 1800 Glycerine ml' 600 Hydrochloric acid, 10% ml 650 Ammoniumalginate, 10% sol m1. 150 Octyl alcohol ml.. 5: Sulphucrex (wettingagent) ml'. Flint powder lb 23 (2) Water ml.- 4530 Gelatine gm 40'Gl-ycerine ml 350 Sodium silicate ml 2000 Hydrochloric acid, 10% ml 950Octyl alcohol ..ml Sulphucrex (sodium lauryl sulphate). ml... L50 Flintpowder lb 32 (3) Ethyl silicate mI 1350 Methylated. spirits ml 3.720Hydrochloric: acid, 10% m1- 250 Water ml 80 Flint powder lb* 23 Sodiumsilicate ..ml 790 Water ml 0: Concentrated nitric acid ml 260 Flintpowder lb 10.-

This; mixture. is 0:44 normal acid and. is made by mix ing some of thewater and acid. and pouring the diluted.

silicate slowly into the acid solution with vigorous stir ring toprevent: it gelling Instead of nitric acid. other mineral acids: such ashydrochloric and'. sulphuric: could beused.

A suitable alkali suspension is the following:

Sodium solicate m1 790 Water ml 17"I0' Flint powder." lbs 10 Thismixture is 0.88' normal alkali;

In the acid and alkali suspension mentioned above the flint powder maybe replaced partly or wholly by other refractories such as finelygroundalumina, sillimanite, zirconia, zircon and the like. a 1

The silicate content of both solutions may be the same and with a gradeof sodium silicate commonly used the final silica content of eachdipcoat liquid will be about 7.5 percent, but the silica content ofthese solutions can be varied and useful results have been obtained in arange from 3 to percent. When an adjustment to the silica content ismade, it is done in such a way that the first solution is always acidand the second always alkaline. The acid normality of the former shouldbe in the range of O.2 to Q.85 normal andthe alkaline normality of thesecond should be in the range 0.4 to 1.7 normal, to preserve the desiredratio of 1;23.

Each layer or coat of the shell is produced by one dip in each separatesolution, each dip being followed by the "stuccoing operation.

Any time up to about one hour may elapse between the dips in the acidand alkali solutions as'long as the former has not dried because it mustbe wet in order that the gelling reaction will follow. The dipping may,however, take place without any interval of time at all and I preferthat that interval be not more than two or three minutes.

. The second and succeeding clips in acid solution can be made at anyconvenient interval after the preceding alkali dip, from A to 3 hoursbeing generally most suitable.

A variety of refractory materials can be used for the stuccoingoperation such as silica, sillimanite, firebrick grog (crushedfirebrick), zirconia, zircon and the like. I prefer firebrick grog.

The grain size of the refractory used is important and the followingrange is suitable:

Percent retained by weight Sieve size each followed by the appropriatestuccoing," until a.

shell about /1 inch thick is produced. (This normally requires 3 or 4clippings in each solution.) The expendable pattern is now removed bycausing it to liquify (by melting or preferably by a suitable solvent).

The shell mould with pattern removed is next placed in a suitable boxand invested with dry sand, care being taken to prevent any loose sandfrom entering the shell mould.

The dry sand will be a sufficient backing up of the shell for normalcasting. If, however, pressure casting is to be used a firmer supportmay be required and in that case the same may be caused to set byincorporating in it a fritting material. Various fritting materials maybe used such as anhydrous sodium silicate, sodium nitrate, sodiumsilico-fluoride. These frit at different temperatures and one should bechosen which will frit at the temperature at which the mould is to beheated.

When the shell has been suitably invested the metal may be poured in (orforced in) and the casting completed.

The advantages of this invention are in the saving of time and expensein comparison with the conventional method, and in the production ofbetter castings. The materials employed are much cheaper thanin theconventional method. The vibrating equipment necessary in that method toconsolidate the investment material is unnecessary. If the pattern is tobe removed by heating it, the heating time required is much less (of theorder of one hour or less as compared with 8 to 10 hours for theconventional method.) Alternatively if the pattern is removed by asolvent the time for removal is reduced owing to the rapid permeation ofthe solvent through the wall of the shell. Finally the shell is muchmore permeable to air than is the mould backed by a solid investment.Air can therefore escape from the mould through the shell when castingis taking place and this enables thinner sections to be cast and alsoavoids faulty castings due to air being trapped in the mould.

I claim:

1. The method of forming a multi-layer shell mould around an expendablepattern which includes preparing two solutions of sodium silicate eachcontaining a finely ground refractory material suspended therein and onebeing alkaline and the other having an acid material added to make thesame acid but of a normality less than the normality of the alkalinesolution, and then applying to the pattern alternately the acid and thealkaline solutions and stuccoing a finely ground refractory material tothe wet shell after each application, said alkaline solution alwaysbeing applied While the shell is still wet with said acid solution.

2. The method of forming a multi-layer shell mould around an expendablepattern which includes preparing two solutions of sodium silicate eachcontaining a finely ground refractory material suspended therein and onebeing alkaline and the other having an acid material added to make thesame acid but of a normality less than the normality of the alkalinesolution forming an initial refractory coat on the pattern according tothe conventional practice of precision casting, then applying to thecoated pattern alternately the acid and the alkaline solutions andstuccoing a finely ground refractory material to the wet shell aftereach application, said alkaline solution always being applied while theshell is still wet with said acid solution.

3. The method of forming a multi-layer shell mould around an expendablepattern which includes preparing two solutions of sodium silicate of asilicate content ranging from 3% to 15% each containing a finely groundrefractory material suspended therein and one being alkaline with analkaline normality ranging from 0.4 to 1.7 normal and the other havingan acid material added to make the same acid with an acid normalityranging from 0.2 to 0.85 normal but of a normality less than thenormality of the alkaline solution forming an initial refractory coat onthe pattern according to the conventional practice of precision casting,then applying to the coated pattern alternately the acid and thealkaline solutions and exposing the Wet shell after each application toa shower of finely ground refractory material said alkaline solutionalways being applied while the shell is still Wet with 1 said acidsolution.

4. The method of claim 3 wherein ground flint powder is the refractorymaterial suspended in said solutions.

5. The method of forming a multi-layer shell mould for precisioncastingwhich comprises: forming an initial refractory coat around an expendablepattern; then alternately applying to the coated pattern an alkaline andan acid solution of sodium silicate, each solution having a finelyground refractory material suspended therein, said acid solution beingof an acid normality less than the alkaline normality of said alkalinesolution, said alkaline solution always being applied while tnc shell isstill wet with said acid solution; stuccoing a finely ground refractorymaterial to the wet shell after applying each solution; after eachsolution has in turn been applied at least three-times causing thepattern to liquify and how out of the shell; and investing theshell indry sand.

6. The method of claim 5 wherein the alkaline normality of the alkalinesolution is in the range of 0.4 to 1.7 normal; wherein the acidnormality of the acid solution is in the range of from 0.2 to 0.85normal; and wherein the alkaline normality of the alkaline solution isat least twice the acid normality of the acid solution. 5

7. The method of claim 5 wherein the sand has incorporated therein amaterial capable of fritting at elevated temperatures.

8. The method of claim 5 wherein each solution is applied by dipping andwherein the stuccoing operation 10 comprises showering dry material onthe wetshell.

References Cited in the file of this patent UNITED STATES PATENTSCollins Aug. 7, 1945 Feagin May 18, 1948 FOREIGN PATENTS Great BritainNov. 7, 1951 Great Britain Apr. 1, 1953

1. THE METHOD OF FORMING A MULTI-LAYER SHELL MOULD AROUND AN EXPENDABLEPATTERN WHICH INCLUDES PREPARING TWO SOLUTIONS OF SODIUM SILICATE EACHCONTAINING A FINELY GROUND REFRACTORY MATERIAL SUSPENDED THEREIN AND ONEBEING ALKALINE AND THE OTHER HAVING AN ACID MATERIAL ADDING TO MAKE THESAME ACID BUT OF A NORMALITY LESS THAN THE NORMALLY OF THE ALKALINESOLUTION, AND THEN APPLYING TO THE PATTERN ALTERNATELY THE ACID AND THEALKALINE SOLUTION AND STUCCOING A FINELY GROUND REFACTORY MATERIAL TOTHE WET SHELL AFTER EACH APPLICATION, SAID ALKALINE SOLUTION ALWAYSBEING APPLIED WHILE THE SHELL IS STILL WET WITH SAID ACID SOLUTION.